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If I am correct, I think you are referring to "cold lap" or properly called incomplete fusion. It is a common problem in mig welding and can be on the underside of the weld or on the leg or toe of the weld. Obviously, if it is on the underside you can't see it. But if it is on the leg or toe, it will look like the weld is just laying on top of the base metal not really fused together. Kinda hard to visualize until you actually see it. Will try to find a picture and post it for you.
It is usally caused by trying to weld too thick of base metal without some preheat or an incorrectly adjusted machine. It is also a very big problem when mig welding aluminum.

I've taken mig courses and have been mig welding a long time but I'm not a "professional welder" in that I don't weld for a living. However, I weld a lot - way more than the average hobbyist and I get a lot of positive feedback on my skills. I manufacture a product which I sell mostly at car shows and swapmeets.

Here's the dilemma ...
On two occasions I've had people tell me "there are some cold joints there" but they've never actually pointed them out. The only thing I can see is that on some welds the surface of the bead is not "shiny". Otherwise, they all look pretty much the same to me. In 5 years of making this product I've had nothing but positive feedback from customers and not a single mention of a broken joint. And many of my products have been sold to fabricators who've felt building the product was not worth the effort versus buying mine!

I will point out that I weld mostly 3/16 material but there are some lighter gauge pieces that I attach without changing the settings. But I can't see this causing cold joints.

Could it be, as one observer pointed out when someone made the comment, that "some people just need something to complain about" ?

Mig cold joint??

Could it be, as one observer pointed out when someone made the comment, that "some people just need something to complain about" ?
U may hv hit the nail on the head ^ . And
Jpence38 had pretty good explanation.
But it pretty hard to get a cold weld on 3/16 metal with out it looking pretty bad.
With that thin of metal if the weld looks good it's probably fine. Dull or shiny isn't a problem & the lighter metal at that setting should be a problem